Many devices use touch screens as a convenient and intuitive way for users to both view and enter information. Common applications include mobile phones, PDAs, ATMs, GPS navigation systems, electronic games, and computer interfaces, to name just a few examples. Touch screens allow a user to interact with a device by using a finger or stylus to touch displayed objects, such as icons, text, buttons, and the like. In some applications, a user may also “write” directly on a touch screen, such as in a PDA or other device that implements character recognition.
In practice, the input function of the touch screen and the output function of the display are typically performed by physically distinct devices. For example, a touch-sensitive device may be attached to the front of a standard display device. The touch sensitive device may detect the location of an object (e.g., a finger or a stylus) touching a screen, while the display device presents images to the user. A control device associated with the touch sensitive device correlates the location of the touch with the displayed images to understand the user's intent.
Polarizer-based touch screens can be very useful in applications where it is desirable to minimize reflection or glare. A polarizer is a device that filters an unpolarized or mixed-polarization beam of electromagnetic waves (e.g., light) to only pass waves with a single polarization state (e.g., a single linear polarization). Polarizers are used in many optical techniques and instruments, and polarizing filters find applications in photography as well as display technologies. Polarizers can be divided into two general categories: absorptive polarizers, where the unwanted polarization states are absorbed by the device, and beam-splitting polarizers, where the unpolarized beam is split into two beams with opposite polarization states, one of which is reflected and one of which is transmitted through the polarizer.
One common type of polarizer is absorptive-type Polaroid® film, which is made from polyvinyl alcohol (PVA) plastic with an iodine doping. Stretching of the plastic during manufacture ensures that the PVA chains are aligned in one particular direction. Electrons from the iodine dopant are able to travel along the chains, ensuring that the film absorbs light polarized parallel to the chains, while light polarized perpendicularly to the chains is transmitted.
Beam-splitting polarizers may work by reflecting unwanted light or by utilizing the birefringent properties of crystals to split off light of the desired polarity, among other ways. Another type of beam-splitting polarizer is a thin-film polarizer, which is created by layering an optical coating on a substrate material such as glass.
Regardless of the type of polarizer used, it may be desirable to apply one or more additional layers to a polarizer in a display device. For example, a polarizer may include a protective coating to guard against damage caused by physical impact or electrical discharges. This is especially true for polarizers used in touch screens, which must be durable enough to withstand frequent physical contact from a finger or stylus. However, adding layers to a polarizer may compromise the overall optical efficiency of a display device.
Polarizer touch screens are often employed in applications where it may be desirable that a touch screen display be readable in a range of lighting conditions from darkness to strong sunlight. Cell phones, PDAs, gas pumps, and ATM machines are all examples of applications that can benefit from polarizing touch screens. Touch screen display devices may also be used with CRTs, OLED displays, LCDs, and plasma displays. In many applications, especially but not exclusively portable devices, it is also desirable to minimize the thickness of a display device, as this factor can contribute to overall size and weight of an electronic device.
Many display devices use one or more polarizers to control how light interacts with or is emitted by the display. For example, an organic light emitting diode (OLED) display may use a polarizer to reduce interference from environmental light sources. Liquid crystal display (LCD) devices generally have front and back polarizers that are typically oriented orthogonally with respect to each other and separated by a field of liquid crystal material. Behind the back polarizer is a backlight or, less commonly, a reflective surface. Images appear on the display when light is transmitted from the backlight through both of the back and front polarizers. As the polarizers are crossed, however, light passing through the back polarizer is aligned so that it will not pass through the front polarizer without first being realigned. To produce images on the display, the liquid crystal material is stimulated according to a desired pattern, causing the polarization of the light to be rotated in places where it is desired to transmit light and leaving other areas dark.
One common touch screen type used with display devices is a resistive touch screen. FIG. 1A illustrates a typical resistive-type touch screen 10 according to the prior art. A glass substrate 12 supports two layers of ITO electrodes 14, 16 separated by an array of dot spacers 18. Seal frits 20 form the outer edges of the touch screen 10, which has a protective layer of PET film 22 on its surface. The resistive touch screen 10 functions by detecting changes in resistance between electrodes on the upper ITO layer 16 and the lower ITO layer 14 when a touch from a finger or stylus deforms the upper ITO layer 16 toward the lower ITO layer 14 at the location of the touch. To form a touch screen display device, the resistive touch screen 10 is laminated onto a display device such as the LCD device 30 of FIG. 1B. A layer 32 of optical clear adhesive (OCA) or double-sided adhesive tape may be used to bind the resistive touch screen 10 to the LCD display 30. The LCD display 30 may include a front polarizer 36, which in turn may have a protective layer (e.g., TAC film or other polymer) on its outer surface (not shown).
FIG. 2 illustrates another type of a display device, namely, a double super twisted nematic (DSTN) type LCD device 50. As the name implies, the device 50 utilizes two liquid crystal cells 66, 68 that include super twisted nematic crystals. Generally, twisted nematic (TN) liquid crystal cells include liquid crystals that are twisted at 90 degree angles. TN liquid crystals cells are widely used in small displays, such as wrist watches, small measuring devices, or the like. This is generally due to their relatively low voltage, low power consumption, and long life span. However, for larger displays and/or better quality displays, a liquid crystal cell that includes a larger twisted angle is required. In this regard, a super twisted nematic (STN) liquid crystal may be used, which may, for example, have a twisted angle somewhere in the range of 180 to 270 degrees.
The DSTN device 50 shown in FIG. 2 includes a liquid crystal display cell 68 that includes two transparent plates 543 and 544 (e.g., glass or plastic plates) arranged parallel to each other. The display cell 68 also includes ITO layers 623 and 624 (or another type of conductive layer) that are fitted on the surfaces of the plates 543 and 544 that face each other. A liquid crystal layer 642 is disposed between the ITO layers 623 and 624. Depending upon the applied voltage to one or more electrodes of the ITO layers 623 and 624, the liquid crystal 642 changes the plane of polarization of the light penetrating through the liquid crystal layer.
The outer surfaces of the device 50 may include a top polarizer 52 and a bottom polarizer 56. The polarizers 52, 56 function to filter the transmitted light so that the light passes in only one plane of polarization. Thus, light beams are transmitted or blocked depending upon the position of the polarizers 52 and 56 with respect to one another and the voltage applied by electrodes of the ITO layers 623 and 624 to the liquid crystal 642, with the result that a corresponding driven pixel of the display appears dark or bright. In this regard, images may be displayed on the device 50 by selectively controlling the brightness of each pixel.
To improve image quality, the DSTN device 50 also includes a liquid crystal compensation cell 66 that may be coupled to the display cell 68 using an adhesive 60 (e.g., an optically clear adhesive (OCA) or double-sided adhesive tape). Like the display cell 68, the compensation cell 66 includes two transparent plates 541 and 542 arranged at a distance from each other. The compensation cell 66 also includes ITO layers 621 and 622 (or another type of conductive layer) that are fitted on the surfaces of the plates 541 and 542 that face each other. A liquid crystal layer 641 is disposed between the ITO layers 621 and 622.
The compensation cell 66 may essentially be similar to the display cell 68, except that the twisted angle of liquid crystal layer 641 of the compensation cell 66 is in the opposite direction of the twisted angle of the liquid crystal layer 642 of the display cell 68. In operation, a linear polarized light beam may be changed into elliptical polarized light when it is passed through the display cell 68. However, the elliptical polarized light is changed back into the linear polarized light while passing through the compensation cell 66, thereby eliminating the tinted color and providing a black and white display.
As described above, touch screen displays are typically formed by overlaying a touch screen device on a display device. A touch screen in this context typically includes a substrate that supports a multi-layered touch sensitive device. This device is typically laminated onto a display device using an adhesive such as optical clear adhesive (OCA). When a separate conventional touch sensitive device is attached in front of a display to create a touch screen, less of the light emitted by the display reaches the user, resulting in a decrease in display contrast. Other disadvantages of overlaying a separate touch sensitive device on a display device include greater overall touch screen thickness, higher cost, and reduced reliability.